Solid state lasers have been found increasingly useful for generating high power laser beams. A solid state laser includes two main parts: a solid laser medium and a pump source. In many applications, the pump source is itself another laser or array of lasers. For example, a GaAlAs diode laser is often used as the pump source for a Nd:YAG laser, since the emission of the diode laser overlaps the absorption band of the Nd ion.
Solid state lasers suffer from a problem, however. The input pump energy inevitably heats up the laser medium. This heating distorts the laser beam for three reasons. First, the index of refraction of the laser medium depends on its temperature, so a temperature gradient within the medium creates a lens, called a "thermal lens". Second, thermal stresses are induced in the laser medium by heating, and these stresses further alter the index of refraction in the direction parallel to the direction of the stress. This directional index change is referred to as "stress birefringence." Third, the laser medium expands when heated and thereby changes its shape. These three effects are undesirable since they distort the optics within the laser cavity.
Solutions to the thermal lensing and stress birefringence problems are known in the art. When the solid laser medium is in the shape of a slab, for example, these problems are ameliorated by causing the laser beam to travel in a zig-zag path within the slab. The zig-zag path moves back and forth across different temperature regions, thereby averaging out the thermal effects to first order. For more information on slab lasers, see for example Walter Koechner, Solid-Sate Laser Engineering, Springer-Verlag (New York, 1976) Sec. 7.3; J. M. Eggleston et al., "The Slab Geometry Laser--Part I: Theory," IEEE Journal of Quantum Electronics QE-20, 289 (1984); and Robert L. Byer, "Diode Laser-Pumped Solid-State Lasers," Science 239, 742 (1988). See also U.S. Pat. No. 3,633,126 issued to Martin and Chernoch, U.S. Pat. No. 4,949,346 issued to Kuper and Rapaport, and U.S. Pat. No. 5,363,391 issued to Matthews, Sorce, and Palombo.
FIG. 1 shows an example of a prior art solid slab laser as disclosed in U.S. Pat. No. 5,479,430 issued to Shine. A laser slab is pumped by a plurality of diode lasers which are positioned above and below the slab. The slab is cooled through the upper and lower faces, the same faces through which the pump light enters. A one-dimensional thermal gradient exists in the vertical direction, with the slab being cooler at the upper and lower faces and warmer in the middle. Since the laser beam zig zags between the upper and lower faces, thermal lensing and stress birefringence effects are nearly averaged out, leaving the beam essentially undistorted.
However, problems still exist with this slab laser. First, the slab is difficult to cool. It is cooled by flowing liquid over the upper and lower faces, but since the slab is cooled and pumped through the same faces, a sophisticated plumbing apparatus must be provided to allow the cooling liquid to flow around the pump optics.
There is another problem with the cooling. The laser beam follows a zig-zag path inside the slab, reflecting from each face by total internal reflection. However, fluid circulates over these faces, and the fluid is heated by the slab. Therefore, local changes in the index of refraction of the fluid occur, causing phase changes in the reflected beam. These phase changes are not uniform across the profile of the beam. Furthermore, if the fluid is not perfectly clean, it can deposit dust or other particles on the reflecting surfaces. These particles can lead to laser radiation damage to the surfaces.
Another problem with the conventional slab lasers concerns a restriction on the power of the output laser beam. There is a limit to the amount of power that may be pumped into the slab because the thermal stresses within the slab can cause the slab to fracture. The maximum power per unit length, (P/l).sub.max, that may be pumped into the slab of FIG. 1 is determined by the laser medium geometry and material parameters and is proportional to the ratio of the width of the slab w.sub.1, to its thickness t.sub.1 : EQU (P/l).sub.max .alpha.w.sub.1 /t.sub.1 (1)
Therefore, if t.sub.1 is decreased, the power input can be increased. However, when t.sub.1 is decreased, the amount of input energy that is absorbed by the slab will decrease since the optical path length of the pump beams in the slab will be shorter. Thus, there is a tradeoff between the amount of power that may be pumped into the slab and the amount of power that the slab can absorb. This tradeoff is a disadvantage, because to optimize the power of the output laser beam, both the pump power and the absorption should be maximized simultaneously.
In an effort to overcome the difficulties with the cooling liquid, Chernoch (U.S. Pat. No. 3,679,999), Kuper et al. (U.S. Pat. No. 4,949,346) and Matthews et al. (U.S. Pat. No. 5,363,391) designed lasers with passive cooling systems, using either a solid or a gas as a heat sink. In these designs, the pump light enters the laser slab through the heat sinks. Therefore the heat sink material must be transparent to the pump light. This requirement severely limits the choice of heat sinks, compromising the range of thermal conductivities available.
Farinas, Gustafson, and Byer (Optics Letters 19 (1994) pp. 114-116) also developed a laser that uses passive cooling by conduction. The laser uses solid heat sinks to cool the front and back faces of the slab of FIG. 1, rather than the top and bottom faces as had been cooled before. Because the cooling faces are distinct from the pump faces, the heat sinks need not be transparent, and can be made from any material with a high heat conductivity, such as metal. This cooling technique avoids the problems of the liquid coolants. However, the plane of propagation of the laser is no longer in the same plane as the thermal gradients. Therefore, the thermal lensing and stress birefringent effects mentioned earlier are not averaged out by the zig-zag path.